System and method for determining reticle defect printability

ABSTRACT

A method and software program for determining printability of a defect on a reticle or photomask onto a substrate during processing. That is performed by creating a pixel grid image having a plurality of individual pixel images showing the defect. A gray scale value is assigned to each pixel image of the pixel grid image and a probable center pixel of the defect is selected. Then the polarity of the defect is determined, with a coarse center pixel of the defect optionally selected using the probable center defect and polarity of the defect. If a coarse center pixel is selected, then a fine center of the defect can optionally be selected from the coarse center pixel and polarity of the defect. From the center pixel the physical extent of the defect can be determined followed by the determination the transmissivity energy level of the physical extent of the defect. Optionally, the proximity of the defect to a pattern edge on the reticle or photomask can be determined using the physical extent and polarity of the defect. Then the printability of the defect can be determined from the transmissivity energy level of the defect and characteristics of the wafer fabrication process being used to produce the substrate from the reticle or photomask.

FIELD OF THE INVENTION

The present invention relates generally to electro-optical inspectionsystems, and more particularly to an automated reticle inspection systemand method for determining which defects in a reticle will print on thesubstrate and effect the performance of a completed semiconductordevice.

BACKGROUND OF THE INVENTION

Present reticle and photomask inspection systems currently identifydefects on reticles and photomasks merely as defective pixels. No effortis made to determine printability and the ultimate impact of identifieddefects on a finalized semiconductor device. That approach has beensatisfactory in the past given the trace widths and number of componentsto be implemented on a single substrate and in a single chip.

However new technology has continued to push the line and componentdensity on a single semiconductor substrate, and in a single chip, togreater and greater levels with ever narrower line widths beingrequired. That being true, and given the previous criteria as to whatdefects are a potential problem, smaller and smaller anomalies inreticles and photomasks are being considered a defect. Given the currenttechnology, anomalies of well below one micron in size (down to 200nanometers in some cases) are being considered defects. Therefore,inspection machines have been refined to detect these ever smalleranomalies on reticles and photomasks.

Currently, in the semiconductor industry, complex reticles andphotomasks that can cost tens of thousands of dollars to produce arebeing scraped since it is believed that even the smallest defect in onereticle or photomask used in the production of a substrate may have adetrimental effect on the performance of the final semiconductorcomponent.

What is needed is a method and system that not only identifies the eversmaller anomalies on a reticle or photomask as a defect, but which goesfurther and considers other characteristics, the location of the defect,and the line patterns on the reticle or photomask, to determine whetheror not each individually identified defective pixel will print onto thesemiconductor substrate. If this is accomplished, many reticles andphotomasks that are currently being scraped could instead be used withno detrimental effect on the operation of the final semiconductorcomponent, thus reducing the cost of production of semiconductordevices. It is believed that the present invention provides thatcapacity.

SUMMARY OF THE INVENTION

The present invention includes a method and software program fordetermining printability of a defect on a reticle or photomask onto asubstrate during processing. That is performed by creating a pixel gridimage having a plurality of individual pixel images showing the defect.A gray scale value is assigned to each pixel image of the pixel gridimage and a probable center pixel of the defect is selected. Then thepolarity of the defect is determined, with a coarse center pixel of thedefect optionally selected using the probable center defect and polarityof the defect. If a coarse center pixel is selected, then a fine centerof the defect can optionally be selected from the coarse center pixeland polarity of the defect. From the center pixel the physical extent ofthe defect can be determined followed by the determination thetransmissivity energy level of the physical extent of the defect.Optionally, the proximity of the defect to a pattern edge on the reticleor photomask can be determined using the physical extent and polarity ofthe defect. Then the printability of the defect can be determined fromthe transmissivity energy level of the defect and characteristics of thewafer fabrication process being used to produce the substrate from thereticle or photomask.

DESCRIPTION OF THE FIGURES

FIG. 1 is a general flow diagram that illustrates the steps of thepresent invention.

FIG. 2 illustrates a 256 by 256 pixel grid image that is used by thepresent invention as a general work area for the present invention, andhere illustrates the determination of the polarity of a defect.

FIG. 3 illustrate a 3 by 3 pixel window that is used to determine thecoarse center pixel.

FIG. 4 illustrates a subpixel peak gray scale value location routine toprform a fine location of the center of a defect.

FIG. 5 is a representative gray scale value variation for a pixel of adefect along one axis with reference to the spacing between the centerpixel of the defect to those pixels extending away from the centerpixel.

FIG. 6 illustrates the determined extent of a defect in the pixel gridimage and adjacent groupings of pixels in the same size and shape as theextent of the defect.

FIG. 7 a illustrates a typical gray scale value variation for the pixelsadjacent to each side of, and those that make up the edge of, a line ona reticle.

FIG. 7 b illustrates the use of linear sub-pixel interpolation to locatethe edge of a line on a reticle.

FIG. 8 is a simplified functional block diagram of a prior art maskinspection system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

There are numerous inspection machines available that have thecapability of identifying defects on a reticle or photomask. An exampleof such a machine that performs the inspection automatically by eitherdie-to-die or die-to-database inspection is described in detail inEuropean Patent Specification EP 0532927B1 published Feb. 21, 1996,entitled “Automated photomask inspection apparatus”, and which isincorporated herein by reference. In performing that inspection, theabove identified inspection machine, and other similar machines, scansthe reticle or photomask and pixelizes the image, saving the pixellocation information for each of the scanned regions where there is notagreement between the dies (in die-to-die) or between the die and thedata base (in die-to-data base). A typical pixel size used by suchinspection machines is a 0.25 μm square. What is not determined by thecurrently available defect detection machines is the transmittibleenergy level of light through the groups of pixels that constitute eachdefect; more specifically the transmittable energy level of theradiation frequency used by the steeper to expose a semiconductor waferto the pattern on the reticle or photomask prior to each chemicalprocessing step of the wafer in the production of the finishedsemiconductor component.

It has been discovered that there are numerous factors that contributeto whether or not a defect on a reticle or photomask will print on asubstrate. The size of such a defect is only one of those factors. Ithas also been determined that the energy level that will pass throughsuch a defect is equally important to being able to make a determinationas to whether or not such a defect will print onto a substrate that isexposed to such a reticle or photomask. There are still other factorsthat contribute to whether or not such a defect will print onto asubstrate.

The primary factor as to the printing of a defect in a reticle on asubstrate is the transmittible energy level through that defect. It isclear that if the defect in question is a type that is not transmissive,there can be no trace of that defect on the substrate exposed by thereticle in which the defect is contained, regardless of the size of thatdefect.

There are numerous other factors that influence whether or not a defectprints onto a substrate. Those include, among other factors, the type ofresist used on the substrate, line width size, stepper type, numericalaperture of the stepper, focus of the stepper, radiation frequency ofthe stepper, exposure time of the stepper, etc.

Referring to FIG. 1, the actual process of the present invention thusbegins with the defect review menu (10) that the prior art inspectionmachine creates. A microscope is then used, by an operator orautomatically, to capture the image of a defect (12) from the defectreview menu by scanning that defect image and creating a pixel gridimage (e.g., 256 by 256 pixels with the pixel size being 0.25 μm) of adefective area. Then, using a gray scale resolution of 256 levels, eachpixel in that captured image is digitized (14) by assigning a gray scalevalue that corresponds to the brightness or darkness of that pixel from0 to 256, with 0 being for opaque pixels and 256 being for fullytransmissive pixels of the defective area of the photomask. FIG. 8 is aprior art mask inspection system (FIG. 1 of European PatentSpecification EP 0532927B1) that could be used to perform this functionwith optical subsystem 116 acting as the microscope, and delivering theimage from the reticle 114 to electronic subsystem 120, all under thecontrol of control computer 124.

More specifically, the simplified block diagram of FIG. 8 is of a priorart mask inspection system 110 that includes a stage 112 for carrying asubstrate 114 to be inspected, an optical subsystem 116, a data baseadaptor 218, an electronics subsystem 120, a display 122, a controlcomputer 124, and a keyboard 126. The stage 112 is a precision devicedriver under control of subsystem 120 and capable of moving thesubstrate 114 under the test relative to the optical axes of the opticalsubsystem 116 so that all or any selected part of the substrate surfacemay be inspected. Optical subsystem 116 includes a light source 130 andrelated optics which cause a beam of light to be deflected back andforth over a small angle as viewed by the substrate 114. The light beamemitted by light 130 is deflected by the combination of twoacousto-optic elements; an acousto-optic prescanner 140 and anacousto-optic scanner 142. When the light beam emerges from the scanner142 it then enters a cube beam splitter 160. The beam next passesthrough an objective lens 182 which focuses the beam onto the substrate114. Light passing through the substrate 114 is then collected by acondenser lens 184 and focused onto the transmission detector 134.

With a gray scale value assigned to each pixel in the defect area, theprobable center of the defect is selected (16) and the coordinates ofthe pixel at that location are noted. Next the polarity (white or black)of the defect is determined (18) by comparing the gray scale value ofthe pixel at the selected probable center of the defect to the grayscale value of at least one reference pixel a number of pixels spacedapart from the probable center pixel (e.g., 10 pixels to the right). Ifthe gray scale value of the selected probable center pixel is less thanthe gray scale value of the reference pixel, the defect is considered tobe black, or have negative energy. If the gray scale value of theselected probable center pixel is greater than the gray scale value ofthe reference pixel, the defect is considered to be white, or havepositive energy.

Alternatively, reference pixels 2, 5, 7 and 10 pixel positions away fromthe probable center pixel could each be checked and if gray scale valuesuccessively from reference pixel to reference pixel continues to dropthen the defect is considered to be white, or have positive energy.Whereas, if the gray scale values successively from reference pixel toreference pixel continues to rise then the defect is considered to beblack, or have negative energy. However, if the gray scale value of thereference pixels at first moves in one direction and then changesdirection the further that reference pixel is from the probable centerpixel, the probable center pixel is near a line edge and the referencepixel progression will have to be performed in another direction withoutencountering a line edge.

FIG. 2 illustrates a pixel grid image 34 as discussed above with respectto blocks 12 and 14 above. Additionally, there is shown a probablecenter pixel 36 of that image and a single reference pixel 38 that isused as discussed above with respect to blocks 16 and 18 to determinethe polarity of the defect. Alternatively, FIG. 2 also shows a probablecenter pixel 36′ and reference pixels 38 ², 38 ⁵, 38 ⁷ and 38 ¹⁰, asdiscussed in the alternative approach that avoids making the decisionwhen there is a line edge in close proximity to the probable centerpixel.

This procedure to identify the defect as either black or white could berefined further by considering a second reference pixel either furtheraway from the selected probable center pixel, or in another directionthan the first reference pixel, if the gray scale differences betweenthe first considered reference pixel and the selected probable centerpixel are closer together than a preselected difference. Still otherdistances and directions could be tried until a more definitive valuedifference is observed to better determine the polarity of the defect.

Referring again to FIG. 1, with the polarity of the defect determined, acoarse center of the defect (20) is determined by finding the pixel inthe defect with the minimum or maximum (according to the polarity) grayscale value. This is accomplished by comparing the gray scale values ofthe pixels in a square pixel window around the selected coarse centerpixel (e.g., 3 by 3 pixels with the selected coarse center pixel in thecenter). If the gray scale value of one of those pixels in comparisonwith the selected pixel is determined to be higher (white polaritydefect), or lower (black polarity defect), that pixel is selected as thenew coarse center pixel and a second pixel window of the same size,centered about the new coarse center pixel, is observed and the searchis performed again. This process can be repeated as many times asnecessary to find a better choice of the coarse center pixel of thedefect. To insure accuracy this test can be repeated at least someminimum number of times, perhaps 5, to fully search for and identify thebest coarse center pixel.

FIG. 3 illustrates the use of a square pixel window 40 of the typedescribed above with respect to block 20 of FIG. 1. Here, for the firststep at the determination of the coarse center pixel with the probablecenter pixel 36 first selected as the coarse center pixel with the firstsquare 3 by 3 pixel window 40 drawn around it. In each of the squares ofwindow 40 a representative gray scale value has been shown with 76having been assigned to pixel 36. Then, the gray scale value of each ofthe surrounding pixels is compared to the value of pixel 36 to determineif there is a pixel that has a gray scale value that is higher than thatof pixel 36. In this example it can be seen that pixel 42 has a grayscale value that is 79 versus the 76 of pixel 36, thus pixel 42 isselected as the next coarse center pixel. Again a 3 by 3 pixel window 44is drawn around pixel 42 and the surrounding gray scale values of thosepixels are compared to the gray scale value of pixel 42 in search ofanother pixel with a higher gray scale value. In this example, pixel 42has the highest gray scale value and therefore would be selected as thecoarse center pixel of the defect.

Returning again to FIG. 1, with the coarse center pixel of the defectdetermined, the subpixel center of the defect can be more finelydetermined (22) by using a subpixel interpolation routine. Using thegray scale values for the best coarse center pixel, and surroundingpixels (e.g., the pixel on either side of the coarse center pixel ineach direction of interest—x, y and diagonals perhaps), a fineapproximation of the defect center, to within less than a pixeldimension (e.g., to within 0.1 pixels) can be determined.

FIG. 4 shows an example of a subpixel interpolation routine in onedirection. Here, the gray scale level variation versus distance for arepresentative defect is shown with the location and gray scale valuesof the coarse center pixel 42 (here numbered 2) and the nearest pixelson opposite sides thereof along the same axis (here numbered 1 and 3,respectively). Also, for purposes of this illustration, pixels 1, 2 and3 each has a gray scale value of 60, 80 and 75, respectively. Also fromthe defect gray scale curve it can be seen that coarse center pixel 42is not quite at the peak gray scale value of the defect along therepresentative axis. The fine center of the defect can be located withthe following formula: $\begin{matrix}{{{fine}\quad{pixel}\quad{center}} = \frac{x_{1} + {2\left( x_{2} \right)} + {3\left( x_{3} \right)}}{x_{1} + x_{2} + x_{3}}} & (1)\end{matrix}$where

-   -   x₁ is the gray scale value of pixel 1;    -   x₂ is the gray scale value of pixel 2; and    -   x₃ is the gray scale value of pixel 3.

Using the sample gray scale values of FIG. 4 a, equation (1) yields:$\begin{matrix}{{{fine}\quad{pixel}\quad{center}} = {\frac{60 + {2(80)} + {3(75)}}{60 + 80 + 75} = \frac{60 + 160 + 150}{215}}} \\{= {\frac{445}{215} = 2.0697}}\end{matrix}$thus the fine center pixel location is 0.0697 of a pixel width closer topixel 3 from pixel 2, or 6.97% of a pixel width from the center of pixel2 in the direction of pixel 3.

Again returning to FIG. 1, with the center of the defect determined, thesize of the defect, or physical extent of the defect in severaldirections (24), can next be determined. This is accomplished by firstnoting the gray scale value of the pixel at the center of the defect.That gray scale value is then compared to the gray scale value of thenext adjacent pixel in a selected direction. If the difference in grayscale values is greater than a preselected level (e.g., 2), the pixellocation is incremented in the same direction by one with the gray scalevalue of that next pixel compared to the previous adjacent pixel. Ifthat difference value is still greater than the same preselected level,that process continues in that same direction until the difference valuedoes not exceed the preselected level. Once the pixel where thedifference value does not exceed the preselected value is determined,that pixel is considered to be the extent of the defect, or on the edgeof the defect, in that direction. The same procedure is performed inother selected directions to similarly find the extent, or edge of thedefect, in each of those directions. This effectively defines the edgeof the defect, or, since the pixels are square, substantially a boxaround the defect. How many directions in which the comparisons areperformed is optional and may be partly dependent on prior knowledge asto the approximate location of the defect relative to other features onthe reticle (e.g., proximity to a region of the opposite polarity suchas a trace and an opaque region, corner of an opaque or transparentregion) of the accuracy to which the extent of the defect is to bedetermined (e.g., it may be desirable to perform the same functiondiagonally outward from the center of the defect, or perhaps radiallyevery 10°).

FIG. 5 illustrates, as a bell shaped curve 48, how the gray scale valuesof the individual pixels of a defect might vary with distance from thegray scale value of the pixel at the fine center, F, of the defect alongone axis. Thus, to determine the extent of the defect the gray scalevalue of adjacent pixels are compared with each other until thedifference in gray scale values between two adjacent pixel along thesame axis from the center pixel, F, is below a preselected thresholdvalue. Using the values shown in FIG. 5 and assuming that the thresholdvalue is 2, the pixel at location 50 will represent the extent of thedefect to the left of the defect center since there is only a differenceof 1 with the gray scale value of the next pixel to the right, whereasthe differences between all other pixels between pixel 50 and the centerpixel are all greater than 2. As stated above, this technique is used inas many other directions as desired to find the extent of the defect inthe pixel grid image 34.

Back to FIG. 1, with the extent of the defect determined it is nowpossible to determine the transmittable energy level of the defect (26).First, the pixel energy of the defect is determined by summing all grayscale values of all of the pixels that are encompassed by the extent ofthe defect in each direction considered above. Second, in order tomeasure the energy difference provided by the defect alone, it isnecessary to subtract an approximation of the background energy valuethat would have been present had there not been a defect, or in otherwords the background noise of this region of the reticle image. Avariation in the transmittible energy level of a defect could resultfrom areas that are totally transparent, to those that are somewhattranslucent, to those that are totally opaque. The causation for thosetypes of variations in transmittible energy level are numerous. Perhapsthe chrome layer on the reticle is thinner in some locations, perhapsthere is a scratch that extends substantially through, or all the waythrough, the chrome layer, perhaps there is a chemical stain on thetransparent or opaque regions on the reticle that may or may not impedethe transmission of light through the transparent regions . . . the listis virtually endless.

One way to approximate the background energy of the defect is to sumtogether the gray scale values for all of the pixels in an immediatelyadjacent region to the pixel grid image (see 12 above) that is the samesize and shape as the determined extent of the defect. For best results,this immediately adjacent region should be defect free, and of the samepolarity as the defect. The summed energy from that adjacent region isthen considered to be approximately what would have been the backgroundenergy level of the defect region and is therefore subtracted from thesummed energy level of the defect region to get a more accurate measureof the transmittible energy level of the defect region.

To obtain a more accurate approximation of the background energy of thedefect region, multiple adjacent regions of the same size and shape canbe used with the energy levels of those regions averaged together. Thenthat averaged energy value would be subtracted from the energy value ofthe defect region. Through the use of the average level, the effects ofsome anomalies or system noise in the regions being used to determinethe background energy level would be reduced.

FIG. 6 illustrates the pixel grid image 34 for the defect of interestwith the extent of that defect shown by outline 52. First the total ofthe gray scale values for all pixels within that defined defect extentare summed together. Then an area of the same size and shape 54 isconsidered adjacent to the extent of the defined defect with the grayscale values of all of the pixels within that area added together todetermine an approximation of the background gray scale energy value forthe defined defect area 52. The value for area 54 is then subtractedfrom the value of defect area 52 to determine the actual level oftransmittible energy level of defect area 52. Alternatively, asdiscussed above, multiple adjacent areas 54, 56 and 58, of the same sizeand shape can also be defined adjacent to defect area 52 with the totalgray scale energy level for all of the pixels within those areas addedtogether and the total then divided by 3 in this example to determine anaverage background energy level to be subtracted from the energy levelof defect area 52.

Referring to FIG. 1, it is also known that the proximity of a defect ina reticle to an edge in the pattern on the reticle can have an amplifiedeffect on the printing of the defect to the substrate. It is necessaryto then determine that proximity, if it exists. The proximity of adefect to a pattern edge on a reticle (28) is then determined bysearching in numerous directions outside the determined extent of thedefect for a gradient (geometry edge) where the gray scale value rapidlyapproaches the opposite polarity of the defect region. Linear sub-pixelinterpolation is then used to determine the 50% point of the gradient(i.e., where the gray scale value is one half the difference in themaximum gray scale levels on each side of that pixel located at thepoint of transition). With the transition pixel location determined, thedistance between the transition pixel and the center pixel of the defectin microns is the distance to the reticle edge from the defect.

FIG. 7 a illustrates the typical gray scale values of pixels that formthe edge of a line on a reticle. In this example pixels P₁, P₂, P₃ andP₄ are shown, respectively, as having a relative gray scale value of afew percent, 30%, 68% and 100%. Further, as stated above, the locationof the edge of a line is defined as where the relative gray scale valueis 50%. Since there is no pixel that has the 50% value, linear sub-pixelinterpolation is used to determine a close approximation to thatlocation. In this example it can be seen that location is somewherebetween the centers of pixels P₂ and P₃. In FIG. 7 b the portion of thecurve that includes the relative values and spacing of pixels P₂ and P₃are shown with a straight line drawn between those two points on thecurve. Thus, since the relative values for those points are 68% and 30%,a difference of 38, and the difference of 50% from 68% is 18, thelocation of the 50% point will be {fraction (18/38)} ({fraction (9/19)})of a pixel width from the center of pixel P₃ to the center of pixel P₂.Thus, the distance from the line edge to the defect center pixel, F, isthe distance from the defect center pixel, F, to the center of pixel P₂plus {fraction (9/18)} of a pixel width.

As stated above, (see FIG. 1) other factors contribute (30) to whether adefect on a reticle prints onto a substrate (e.g., type of resist, typeof stepper, illumination frequency, etc.) with different weightingfactors being assignable for each of those variables once it is knownwhat chemicals and equipment a manufacturer uses. Thus, isolated defectprintability is predicted by applying selected weighting to the energyof the defect where those weighting factors are attributable to aparticular wafer fab process. Similarly, near edge defect printabilityis also determined by both that distance and the particular wafer fabprocess that is used. Thus, other weighting factors must be applied tothe energy level of the defect to predict printability of those defectsthat are near an edge. There are therefore two factors that worktogether to determine the near edge weighting factor to use: how close adefect is to an edge with a higher weighting value necessary the closerthe defect is to the edge; and the particular wafer fab process beingused.

It should be noted that the above discussion has been for a singledefect, and it should further be understood that for multiple defectsthat may be found in a reticle the above described procedure would berepeated for each such defect that was not otherwise incorporated intothe defect extent of an earlier processed defect.

It should further be noted that the above discussion has included agroup of procedures, with some of those procedures being optimizationprocedures, and that if some of those procedures are not performed,improvement over the prior art will still be achieved. For example,those procedures corresponding to blocks 20, 22, 28 and 32 are secondaryprocedures that can be omitted with a useful result still beingachieved.

While the present invention has been described having several optionalsteps, it is contemplated that persons skilled in the art, upon readingthe preceding descriptions and studying the drawings, will realizevarious alternative approaches to the implementation of the presentinvention, including several other optional steps, or consolidations ofsteps. It is therefore intended that the following appended claims beinterpreted as including all such alterations and modifications thatfall within the true spirit and scope of the present invention.

1. A computer program stored on a computer-readable medium fordetermining the printability of a defect on a reticle or photomask ontoa substrate during processing of said substrate, said printability beingdetermined from a defect review menu of said reticle or photomaskprepared by an inspection machine and weighting factors related to afabrication procedure used to produce said substrate, said computerprogram comprising: a. instructions for creating a pixel grid image of aportion of said reticle or photomask containing said defect identifiedin said defect review menu, said pixel grid image having a plurality ofassociated individual pixel images of said reticle or photomask; b.instructions for assigning a gray scale value to each of said associatedindividual pixel images of said pixel grid image; c. instructions forselecting a probable center pixel of said defect in said pixel gridimage; d. instructions for determining a polarity of said defect; e.instructions for determining a region of physical extent of said defect;and f. instructions for determining a transmissivity energy level ofsaid region of physical extent of said defect. 2-25. (canceled)